1. Field of the Invention
The present invention relates to a nondestructive inspection method and nondestructive inspection apparatus for inspecting the generation status of residual stress nondestructively in a steel material on which a treatment of shot peening was processed, more particularly relates to a nondestructive inspection method and nondestructive inspection apparatus appropriate for the inspection of surfaces processed by shot peening with fine particle shots with a diameter of tens of μm.
2. Description of the Prior Art
When a surface treatment of a steel material to be treated is performed by shot peening, plastic deformation of the surface of the material to be treated is implemented by a collision of shots and the result thereof; residual stress is generated.
This residual stress is a factor determining strength of the material to be treated, particularly bending strength, torsion strength, bending fatigue strength, torsion fatigue strength, and wear resistance. One purpose of shot peening is to modify a surface through generation of residual stress, therefore, an inspection of the state of generation of residual stress is one of important indices for quality control.
There are two known general methods for measuring the residual stress: a destructive method in which dimensional changes due to a release of the stress are accurately measured while removing an object to be treated which is processed by shot peening successively and a nondestructive method using X-ray analysis.
The destructive method among these methods measures residual stress by measuring deformation of the material to be treated due to reduction of a layer to be processed (residual stress generation layer) by removal of the surface of the material to be treated through electropolishing or chemical polishing, but the residual stress generated by shot peening, particularly by shot peening using fine particle shots appears only in a very shallow part near the surface of the material to be treated and may not be measured occasionally in the destructive method.
On the other hand, in the nondestructive inspection method using X-ray analysis, residual stress can be measured without destruction of the material to be treated, but an X-ray valid penetration depth is only several μm from the surface, so a state of residual stress can be inspected only at an uppermost surface of the material to be treated, but cannot be inspected in a depth direction.
To measure a distribution of residual stress in the such depth direction, it is necessary to calculate, under assumption based on material mechanics, the residual stress from deformation of the material to be treated that depends on changes in the cross section area (thickness) of the material to be treated by such as chemical polishing or cutting in the destructive method. However, as described above, in shot peening, particularly in shot peening using fine particles, residual stress generates in a relatively shallow part from the surface, so it may not be measured by the destructive method. The assumption based on material mechanics used in the above method cannot reproduce the state of stress in the material to be treated which should be measured with high fidelity, and the precision is reduced in that measurements are made while changing the thickness of the material to be treated as well as residual stress.
To improve the reliability of the measurement results, “a window method”, which measures a state of residual stress in the depth direction of the material to be treated by X-ray analysis, is suggested.
The window method measures residual stress in the depth direction by X-ray analysis while removing a minute circular or square area called a window instead of the entire surface of an inspection target through chemical polishing or electropolishing; this method can measure the distribution of residual stress in the material to be treated without changing the distribution.
According to the window method, a distribution of residual stress can be measured in the depth direction of the material to be treated without being restricted by the X-ray valid penetration depth and measurement results are accurate, further, the reliability is improved.
However, this inspection of a state of residual stress by X-rays requires destruction of the material to be treated (even if it is a very small or minute opening), so the whole products cannot be inspected and the inspection must be carried out as sampling inspection.
To measure the distribution of the residual stress in the depth direction of the material to be treated according to the window method, it is necessary to repeat the procedure in which the window part is polished until it is reached to a specified depth and then an X-ray inspection is carried out, thereby a lot of works and times are required therefor.
In consideration of the problems of conventional methods of inspecting residual stress, as an inspection method for the material to be treated having a surface processed by shot peening, it is desired to develop an inspection method and inspection apparatus that can easily inspect the state of generation of residual stress in the depth direction in a relatively short time without destruction of the material to be treated; and if such an inspection method and inspection apparatus of this type are realized, the whole products can be inspected.
However, there is currently no inspection method and inspection apparatus of this type, thus a sampling inspection must be performed and if a prescribed faulty rate is generated in the sampling inspection, the whole products manufactured on the line is discarded for quality control, for example. In this method, however, yields are reduced because correctly processed products are also discarded, and incorrectly processed products may be included even if the faulty rate falls within an allowable range.